The present invention generally relates to a transmitter circuit used in a portable radiotelephone, portable wireless telephone or the like. More particularly, the invention is concerned with a transmitter circuit having such a circuit configuration which is suited advantageously for the power control of a power amplifier having two voltage lines of positive (plus) and negative (minus) polarities for the output power control.
In the portable radiotelephone or portable wireless telephone (hereinafter collectively referred to as portable radiotelephone) known heretofore, a power amplifier and a driving amplifier for driving the same are realized by using bipolar elements of Si (silicon). In recent years, however, there has been an increasing tendency of employing power amplifiers which are realized by using a GaAs-FET having a high power conversion efficiency with a view to miniaturizing the portable radiotelephone to obtain light weight.
However, the GaAs-FET involves high expenditure when compared with the Si-element, and it is not only uneconomical but also unreasonable to exclusively use the GaAs-FETs for both the driving amplifier and the power amplifier.
Now representing the additive efficiency of the driving amplifier by .eta..sub.d, that of the power amplifier by .eta..sub.p and the overall or combined additive efficiency of the driving amplifier and the power amplifier by .eta..sub.t, the following relation applies: ##EQU1## where Gd represents the gain (dB) of the driving amplifier and Gp represents that of the power amplifier.
FIG. 5 of the accompanying drawing illustrates graphically a relation between the additive efficiency .eta..sub.d of the driving amplifier and the overall or total additive efficiency .eta..sub.t. As can be seen from the figure, the additive efficiency .eta..sub.d of the driving amplifier presents substantially no contribution to the total additive efficiency .eta..sub.t, when the gain of the power amplifier is higher than that of the driving amplifier and when the additive efficiency .eta..sub.d of the driving amplifier is higher than 30%.
For the reason described above, it is considered unnecessary to use the GaAs-FET elements for both the driving amplifier and the power amplifier. In other words, there can be conceived such an approach that the conventional Si-element is used for the driving amplifier while the GaAs-FET having a high power conversion efficiency is used for the power amplifier which has a significant influence to the overall additive efficiency .eta..sub.t.
By the way, in the cellular radio system inclusive of the portable radiotelephone and the movable phone system, the output power of the ratiotelephone must be controlled in response to a command sent from a base station in dependence on the operating state of the radiotelephone (e.g. in dependence on the distance from the base station).
It is now supposed that an N-MOSFET of Si is used for the driving amplifier with the GaAs-FET being used for the power amplifier.
In this case, as the means for controlling the output power of the radiotelephone, there can be mentioned among others (1) a method of controlling a gate voltage of either one of the driving amplifier and the power amplifier (first method), (2) a method of controlling the gate voltages of both the driving amplifier and the power amplifier (second method), (3) a method of controlling a power source voltage for either one of the driving amplifier and the power amplifier (third method), and (4) a method of controlling the source voltages for both the driving amplifier and the power amplifier (fourth method).
Considering the four control methods mentioned above in comparison with one another, it is noted that the first method can not assure the power control in a satisfactory manner. The third and fourth methods are disadvantageous in that the power source voltage control involves high power consumption in a power control circuit provided to this end. The second method is most preferred because it can ensure sufficient power control with a low power consumption in the power control circuit.
FIG. 6 of the accompanying drawings shows control characteristics of the driving amplifier realized by using a N-MOSFET, and FIG. 7 shows a control characteristic of the power amplifier realized by using a GaAs-FET. As can be seen from these characteristic curves, the gate voltage of an N-MOSFET constituting the driving amplifier is a positive or plus voltage in a range of 0 to about 5 volts.
On the other hand, the gate voltage of a GaAs-FET of the power amplifier is a negative or minus voltage which must be variable within a range of -4 to -3 volts.
Consequently, the power control circuit has to be imparted with a function capable of regulating simultaneously both the positive or plus control voltage and the negative or minus control voltage.
A typical example of the transmitter circuit of a radiotelephone known heretofore in which the driving amplifier and the power amplifier are all realized by using the Si-elements is disclosed in JP-A-62-163407 in FIG. 1 which is annexed hereto as FIG. 4.
The transmitter circuit shown in FIG. 4 operates in the following manner. FIG. 4 shows a part of a transmitting part of a cellular wireless telephone, and a signal from a voltage controlled oscillator 31 is amplified to be a predetermined output by a transmitting power amplifier 32 and is radiated in the air by an antenna 34. At this time a part of the output of the transmitting power amplifier 32 is rectified by a detector diode 37 to be subsequently supplied to a logarithmic conversion circuit 56 having the output connected to a minus input terminal k of a comparator 39. On the other hand, a three-bit signal which represents an output power level control signal sent from a base station is converted into an analog DC signal through a digital-to-analog or D/A converter 40 to be subsequently applied to the other terminal i of the comparator 39. By comparing the voltages applied to the terminals i and k, the comparator 39 controls the bias of a transistor 41 to thereby control the gate bias of an amplifying element 33 (an FET in this case) constituting a part of the transmitting power amplifier 32 for thereby controlling the transmitting power. When the transmitting power increases beyond a set value, potential at a terminal a rises while the potential at the output point p of the logarithmic conversion circuit 56 is lowered. As a consequence, potential at a circuit point j rises with the gate bias potential of the FET 33 being lowered, resulting in that the gate bias of the FET 33 is lowered, whereby the amplification factor of the FET 33 and hence the transmitting power are caused to lower.
As will be understood from the above description, although the transmitter circuit in which the driving amplifier and the power amplifier are both realized by using the Si elements is certainly inexpensive, it not only suffers from degradation in the power conversion efficiency but also requires a heat dissipation or sink structure of a large size because of very large amount of heat generation, thus presenting an obstacle in implementing the portable radiotelephone in a reduced size with a low power consumption. When the driving amplifier and the power amplifier are realized by using the GaAs-FET elements, the portable radiotelephone becomes very expensive, although it can enjoy a high power conversion efficiency.